Unless indicated otherwise all temperatures herein are in degrees Celsius.
Thermoplastics are compositions which can be molded or otherwise shaped and reprocessed at temperatures above their melting or softening point. Thermoplastic elastomers are materials which exhibit both thermoplastic and elastomeric properties, i.e. the materials can be processed as thermoplastics but have physical properties common to elastomers. Shaped articles may be formed from thermoplastic elastomers by extrusion, injection molding or compression molding without the time-consuming cure step required with conventional vulcanizates. Elimination of the time required to effect vulcanization provides significant manufacturing advantages. Further, thermoplastic elastomers can be reprocessed without the need for reclaiming and, in addition, many thermoplastic elastomers can be thermally welded.
Non-polar rubbers, e.g. polybutadiene, random, graft and block copolymers of styrene and butadiene, EPDM rubber, natural rubber, polyisoprene and the like, are readily mixable with non-polar thermoplastics such as polypropylene, polyethylene and polystyrene. Non-polar, highly unsaturated rubbers are generally not used at temperatures above about 125.degree.; and non-polar thermoplastics have low melting points, e.g. about 120.degree. for crystalline polyethylene, about 170.degree. for crystalline polypropylene and about 105.degree. for polystyrene. Thermoplastic elastomers based on non-polar, unsaturated rubbers and thermoplastics generally comprise stabilizers to achieve desired properties in high temperature applications. For instance, known thermoplastic elastomers, e.g. as disclosed in U.S. Pat. Nos. 4,104,210; 4,130,535 and 4,311,628, based on blends of diene or EPDM rubber and polyolefins are generally used at temperatures below about 120.degree..
Non-polar rubbers are generally used in applications free from extended exposure to solvent-like fluids such as automotive transmission fluid, motor oil, antifreeze, etc. to avoid swelling and the possible resulting reduction in performance properties. Resistance to such swelling, especially for application temperatures below about 125.degree., can be achieved by use of polar rubbers, e.g. nitrile rubber, chlorinated polyethylene rubber, neoprene, and the like. Because polar rubbers are not generally miscible with non-polar thermoplastic polymers such as polypropylene, it is commonly necessary to provide compatibilization. See, for instance, U.S. Pat. No. 4,555,546 which discloses blends of polyolefins, cured acrylic ester copolymer rubber and graft copolymer having polyolefin compatibilizing segments and rubber compatibilizing segments.
Alternatively, as disclosed in U.S. Pat. No. 4,141,863 polar rubbers can be blended with polar thermoplastics, e.g. polyamides, polyesters, polyacrylates, polycarbonates, etc. Many of the more desired polar thermoplastic polymers melt at high temperature, for instance nylon 6 melts at about 225.degree.. Since many preferred polar rubbers may tend to degrade when melt blended for extended periods with high melt temperature polar thermoplastic, resulting thermoplastic elastomer compositions may not have optimal properties.
Fortunately, certain polar acrylate rubbers are exceptionally heat stable and can be useful in blends with thermoplastics. Such rubbers include acrylic ester copolymer rubbers, including ethylene/alkyl acrylate ester copolymer rubbers and functionalized, e.g. acid-modified, derivatives thereof. See, for instance, U.S. Pat. Nos. 4,310,638; 4,327,199; and 4,473,683 which disclose blends of polar thermoplastic polymers, i.e. polyamides, polyesters and styrenic polymers, respectively, with acid-containing acrylic ester copolymer rubber that is neutralized, i.e. ionically crosslinked with magnesium or zinc oxide. While such blends exhibit enhanced thermal stability, they are susceptible to solvent swelling, especially at higher temperatures where the ionic crosslink bonding becomes labile.